Generation of tar is a ubiquitous problem in production of fuel and synthesis of gas from biomass. Some gasifier systems produce gases high in tar concentrations, whereas others produce gases low in tars. Depending on the application of the produced gas, different extents of tar removal are required. For example, burning the produced gas in some industrial units, such as glass melting furnaces, can tolerate higher tar concentrations, than can other applications, such as lime kiln burners, which require much lower tar content in the gas they burn. Using the gas to produce electricity, such as in a gas turbine or a gas engine, requires that tar concentrations must be significantly lower than in most industrial burners. The most demanding specifications with regards to tar content are for catalytic conversion of syngas, where gas must contain extremely low concentrations of tars.
Thus, when producing fuel gas from biomass, it is usually desired to remove the tar. When producing synthesis gas, tar reduction is essential. Tars can be either captured and removed from the gas by physical methods, or converted to useful components by chemical reactions.
Physical methods for removal or reduction of tar include tar capture through scrubbing the gas with water, as described in US Patent publication 2006/0265954 (incorporated herein by reference) or as practiced by FERCO (Paisley M A et al., Proc. ASME Turbo Expo., June 2001), or with oils or other liquids as described in U.S. Pat. No. 4,497,637 (incorporated herein by reference). Physical methods also include passing the gas through an electrostatic precipitator or a filter, which have varied efficiency (Milne T A et al, NREL Report NREL/TP-570-25357 (1998), Han & Kim, Renewable & Sustainable Energy Reviews 12:397-416 (2008)).
Chemical methods for removal or reduction of tar are also commonly known. These methods are typically based on thermal reactions of cracking tar into lighter compounds at high temperature (for example, the NREL Thermal cracker, which operates at about 1300° C.), steam reforming of tar in the presence of catalysts (as described in U.S. Pat. No. 5,213,587 and US patent publication 2008/0244976, both incorporated herein by reference), or oxidizing the tar at high temperature (as described in US patent publication 2009/0090053).
In the thermal reaction methods, in general, provided that sufficient residence time is allowed for the reactions to occur, the higher the temperature experienced by the gas, the lower the resultant tar content. Assuming that the temperature is sufficiently high for the reactions to occur, the longer the residence time experienced by the gas, the lower the resultant tar content.
There are two generally practiced ways of thermally treating the gas. Thermal treatment can occur directly in the gasifier, by maintaining the gasifier at very high temperatures (referred to herein as Primary Thermal Treatment). Thermal treatment can also occur in a secondary vessel, referred to herein as Secondary Thermal Treatment. For example, the temperature in the gasifier can be heated to a moderate gasifier temperature, such as 800° C., then gas can be transferred to a secondary vessel for heating to a much higher temperature (for example, 1200° C.).
In the catalytic reaction methods, in general, at a given temperature and time of contact, tar content will be lower in the presence of certain catalysts than in their absence. Longer contact times between the gas and the catalyst, and higher temperatures, generally increase tar conversion (i.e. lower tar content in the gas), though there are usually optimum conditions above which catalytic processes decrease in efficiency.
Many catalysts useful for the catalytic reaction methods are known in the art (see, for example, Han & Kim, Supra and Xu et al, FUEL in press 2010, both incorporated herein by reference). Catalysts are generally classified as either low-cost, natural, or throw-away catalysts such as dolomite, or high-cost, engineered catalytic materials such as nickel-based naphtha stream-reforming catalyst, marketed and sold by chemical catalyst companies for the specific purpose of removing tar from gas or other biomass-derived fuels.
There are two generally practiced ways of adding catalyst. Primary catalytic treatment involves adding catalyst directly to the gasifier. Thus the catalyst is present as the biomass particles are heated up, as they pyrolyse into char, gases and tars, and as the char is gasified. The produced tars undergo the tar cracking and/or reforming in situ. In secondary catalytic treatment, the catalyst is placed in a second reaction vessel, downstream of the gasifier. Thus, the biomass particles are heated up, and pyrolyse into char, gases and tars, and the char is gasified in the gasifier; the gases and tars are then transferred to a second reaction vessel, which contains (or to which is added) catalyst, and where the tar cracking and/or reforming is performed.
Much research has focused on the effects of different materials as catalysts, and on their relative efficiency when applied in primary or secondary treatments. Generally, secondary catalytic treatment is more costly in terms of both capital and operating costs, because it requires an extra vessel and costs for its operation (including heating costs, etc.). However, it can result in greater extent of tar removal as compared to primary treatment. Both primary and secondary treatments can be used together, typically with better results than either type of treatment on its own.
The selection of an appropriate tar reduction treatment depends on a weighing of the disadvantages of each method. Primary thermal treatment in a typical dual-bed steam gasifier requires hotter combustor temperatures, or greater extents of char combustion, in order to increase the gasifier temperature. This requires more fuel to the combustor, and less to the gasifier, reducing the efficiency of the gasification process. On the other hand, secondary thermal treatment requires a second vessel to superheat the gases, resulting in increased capital and operating costs. In primary catalytic treatment, catalyst costs are high, and catalyst can readily degrade by carbon deposition, or by attrition, when present in the gasifier bed. Make-up catalyst is also expensive. Secondary catalytic treatment requires a secondary tar reforming vessel (see for example US 2008/0244976 and U.S. Pat. No. 5,213,587), resulting in additional capital cost. Often, the reforming vessel is very large, and can cost more to build and operate than the gasifier itself (See NREL Report 2007).